1. Field of the Invention
The invention relates to a pulse compression system. More specifically, the invention relates to such a system which includes a generator for generating for transmission, frequency or phase modulated pulses, each pulse consisting of two or more sub pulses each of which is modulated by a different pulse compression code for detecting targets at short range and long range, the compression codes having low cross-correlation properties, the sub pulses having the same centre frequency, one of the sub pulses being substantially longer than the other sub pulse.
2. Description of Prior Art
Traditionally, because of limited peak transmission power, pulse compression systems with relatively high compression ratios, and therefore long frequency or phase modulated pulses, have been used for long range detection in radar systems. The pulse compression code allows the range resolution performance of a short pulse to be retained; however, these systems are limited in their ability to detect targets at short ranges since the radar is usually unable to receive the reflected pulse until transmission has ceased. To overcome this limitation, a system described in U.S. Pat. No. 4,136,341, Mulder et al, Jan. 23, 1979, was introduced in which a second pulse is generated and the two different pulses are transmitted. While the first pulse is phase or frequency modulated, the second pulse is not modulated and its length is optimized to ensure adequate detection at range not covered by the long modulated pulse. The two pulses are separated in frequency (centre frequency of the modulated pulses) to ensure isolation and thus two separate channels and a combiner are required in the transmitter; and a splitter and two separate paths are required in the receiver. A second variation of this system is described in U.S. Pat. No. 4,524,361, Teulings, June 18, 1985, which modulates the short pulse for improved range resolution at the shorter ranges. The centre frequencies of the two modulated pulses are still different to ensure isolation of one pulse from the other in the receiver channel.
The prior art has further teachings relating to pulse compression systems as, for example, in U.S. Pat. Nos. 3,852,746, Lowenschuss et al, Dec. 3, 1974; 3,860,926, Gillmer, Jan. 14, 1975; 4,096,478, Chavez, June 20, 1978; 4,153,900, Novak et al, May 8, 1979; 4,404,562, Kretschmer, Jr. et al, Sept. 13, 1983; 4,490,720, Kuntman, Dec. 25, 1984; 4,521,779, Lewis, June 4, 1985; 4,524,362, Lewis, June 18, 1985; 4,524,363, Kretschmer, June 18, 1985; 4,560,961, Kestenbaum, Dec. 24, 1985; 4,626,853, Lee et al, Dec. 2, 1986.
The '746 patent teaches an apparatus for generating a modulation signal in the transmitter of a pulse compression system, and specifically, a chirp modulation signal. The '926 patent relates to improved expansion and compression devices for transmitter pulses and receiver pulses of pulse compression systems.
The '478 patent teaches an MTI pulse compression system wherein two side by side pulses of equal length (see FIG. 2b of the patent) are transmitted. The first pulse is compression coded with an up chirp and the second pulse is compression coded with a down chirp. However, because the pulses are of equal length, the system in the '478 patent can be used only for a single range. In addition, the compressed pulses are substracted in the I.F. differencer (28 or FIG. 1 of the patent).
The '900 patent discloses two complimentary phase-coded signals which are synchronously transmitted in a pulse compression system such that, the two autocorrelation functions, when combined, are mutually cancelling except at the synchronous occurrence of the maxima of the autocorrelation functions. A doppler tolerant pulse compression system is described in the '562 patent. An analog-type linear FM modulated transmission pulse is used, and its echo is processed by baseband sampling at the Nyquist sampling rate and then converted to IF.
The '720 patent relates to a radar system where long and short pulses are alternately transmitted. However, this system does not use pulse compression techniques.
The '779 patent relates to a decoder for a pulse compression system. Both the '362 and '863 patents teach pulse compression systems using a fast Fourier transform circuit in a matched filter. In the '961 patent, a system for generating continuous waves useful in pulse compression systems is described. In the '853 patent, a processor for a pulse compression signal useful in a pulse compression system is described. The object of the processor is to enhance the resolvability of the returns.